1. Field of the Invention
The invention relates to an apparatus comprising at least one input terminal adapted to be electrically connected to an external conductor, a ground terminal, and a protection circuit connected between the at least one input terminal and the ground terminal. The protection circuit comprises at least one transient voltage suppressor and a switch for interrupting the electrical continuity of the protection circuit, at least the protection circuit and the switch being accommodated in a housing.
2. Description of the Related Art
Such an apparatus is known and has been sold by applicant. The known apparatus is a facsimile apparatus with type number HFC4 in which an interface unit is conceived to take care of the communication between the facsimile electronics (which are accommodated in a signal processing unit) and an external conductor which is formed by the telephone line to which its input terminal can be connected by a standard telephone plug. A power supply unit serves to provide the interface and signal processing units with electric power. It is possible for the telephone line to carry transient voltages that may be caused, for example, by lightning. The transient voltage suppression means of the protection circuit serve to divert such transient voltages to the ground terminal in order to prevent damage to the apparatus. These means may comprise elements that become electrically conductive when a predetermined voltage across their terminals is exceeded. Examples of such elements are varistors, Zener diodes and spark gaps.
In the known apparatus non-galvanic coupling means (for example transformers or optocouplers) are provided between the interface unit and the signal processing unit and between the interface unit and the power supply unit because, in operation, it is not allowed for a direct electrical connection to exist between the telephone line on the one hand and the power supply unit or the signal processing unit on the other hand. Only signals are allowed to be exchanged between these components. The reason for this is that the telephone network has a ground of its own that is usually floating with respect to the ground terminal of the power supply unit,the latter usually being connected to the safety ground of the electric mains. Therefore, it is necessary to test the isolation between the combination of input terminal and interface unit on the one hand and the power supply and signal processing units on the other hand. This test should be performed before the apparatus leaves the factory and also at regular intervals during the life of the apparatus, for example after repairs have been made. A convenient and generally accepted test consists in providing a high voltage (for example 2000 to 2500 volts) between every input terminal and the ground terminal. If the non-galvanic coupling means provide sufficient isolation, no current will flow during this test. However, such a high voltage would, of course, cause the transient voltage suppression means to become electrically conductive so that the test would indicate a failure of the isolation. To prevent this, means have been provided for interrupting the protection circuit before performing the high voltage test. For this purpose, the known facsimile apparatus HFC4 comprises a male connector soldered to a printed circuit board (PCB) and a female connector cooperating with the male connector. However, the male and female connectors can only be connected or disconnected after the housing has been opened. Moreover, due to the lack of space in the housing it is difficult to handle the female connector.